1. Field of the Disclosure
The present disclosure relates to a motor protection relay and a method for starting a motor of the motor protection relay, and particularly, to a motor protection relay capable of softly starting a motor by variably controlling start power for starting the motor to the motor through controlling of the relay itself and supplying the variably controlled start power to the motor, and a method for starting a motor thereof.
2. Background of the Disclosure
When a motor is started, a large amount of torque is required to rotate the stopped motor at an initial stage, and thus, a start current is very large and when a motor is frequently started, a large amount of electric power is required due to a start load.
When a stopped motor is rotated, if the stopped motor may be rotated by lowering a start load, electric power may be reduced and motor loss may be prevented. Here, as a motor capacity is large, more start load and motor loss are generated. Thus, in order to drive a motor having a large capacity, a motor start device is added to outside of a protection relay to enable the motor to be slowly started to thus protect the motor. However, the related art method uses only ON/OFF start schemes by applying several sequences to an output power source.
FIGS. 1 through 3 illustrate a related art motor starting method.
FIG. 1 is a view illustrating a concept of line-start type power supply and starting.
FIG. 2 is a view illustrating a form of power supply according to the line-start scheme illustrated in FIG. 1.
FIG. 3 is a view illustrating a concept of Y-D start type power supply and starting according to the related art.
The related art line-start scheme is generally used in a load of 15 KW or less when a 3-phase motor is started, and a start force is the largest with an initial start current as 400%. Since the start current is large, the related art line-start scheme is generally used in small equipment and installation cost thereof is low. The related art light-start scheme is a scheme in which an output contact is operated as ON for starting a motor to allow a current to directly flow to the motor to start the motor as illustrated in FIG. 1.
However, as illustrated in FIG. 2, the form of power supply according to the scheme has a problem in that the motor is burdened since start power is temporarily supplied to the motor.
In the Y-D starting as illustrated in FIG. 3, 6 lines are connected, and, at the first time, a motor is started with Y (small voltage), and when a motor inertia is generated, the small voltage is changed to 88D (large voltage) to normally rotate the motor. An initial start current generally ranges from 15 kW to 45 kW, and even larger initial start current may also be used, and an initial instantaneous start load is about 340%. The Y-D scheme reduces a current to ⅓ and allows for smooth starting, compared with the line-start scheme. However, since a start force is ⅓, the Y-D start scheme is used for facilities (fans, low torque motor, and the like) having a small start load.
In addition, a reactor start scheme is also frequently used. A motor is first operated at a low voltage with low torque, and when a motor inertia is generated, a normal voltage is applied to perform a normal operation. An initial instantaneous start load is about 200%, and the reactor starting scheme is applied when a current is 30 kW or greater. Installation cost is expensive more than the line-start scheme or the Y-D scheme.
An automatic reactor start scheme uses 0V to normal voltage. At an initial instant of change in an analog manner, a start load is 100%, providing an optimal start scheme, but installation cost thereof is expensive.
Among the several start schemes, the most economical starting scheme is the Y-D starting scheme, and as a starting condition of the Y-D starting scheme, an operation connection of a 3-phase motor should be a star, that is, Y connection.
By doing that, using a delta circuit during a start time, a phase voltage of 1/√3=0.577 times may be applied to start the motor, and after the motor is started, the connection may be changed into the star connection to operate the motor.
The motor does not have a separate delta or star terminal but a 3-phase coil composed of three groups is instantaneously changed to a delta connection or a star connection by using a magnet and a timer. For the delta-star starting, 3-group 3-phase magnet contactor is necessary.
Also, there is a scheme of softly starting the motor by controlling a current or a voltage by using an external soft starter. This scheme, however, is disadvantageous in that installation cost is increased due to the addition of the external soft starter, compared with other starting scheme (the line-start scheme and the Y-D start scheme).
That is, the related art motor starting techniques have the greatest problem with the implementation itself of the configurations for starting a motor.
For example, for soft starting, an additional device is required, a configuration is complicated, or a great amount of installation cost is incurred, and thus, it may be difficult to apply a technique for starting a motor or its implementation itself may be difficult.
In order to stably and effectively start a motor, soft starting of a motor is essential, and thus, a scheme for solving these limitations is required.